Reference Guide

86 | 802.1ag
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Ethernet CFM
Ethernet CFM is an end-to-end per-service-instance Ethernet OAM scheme which enables: proactive
connectivity monitoring, fault verification, and fault isolation.
The service-instance with regard to OAM for Metro/Carrier Ethernet is a VLAN. This service is sold to an
end-customer by a network service provider. Typically, the service provider contracts with multiple
network operators to provide end-to-end service between customers. For end-to-end service between
customer switches, connectivity must be present across the service provider through multiple network
operators.
Layer 2 Ethernet networks usually cannot be managed with IP tools such as ICMP Ping and IP Traceroute.
Traditional IP tools often fail because:
• there are complex interactions between various Layer 2 and Layer 3 protocols such as STP, LAG,
VRRP, and ECMP configurations.
• Ping and traceroute are not designed to verify data connectivity in the network and within each node in
the network (such as in the switching fabric and hardware forwarding tables).
• when networks are built from different operational domains, access controls impose restrictions that
cannot be overcome at the IP level, resulting in poor fault visibility. There is a need for hierarchical
domains that can be monitored and maintained independently by each provider or operator.
• routing protocols choose a subset of the total network topology for forwarding, making it hard to detect
faults in links and nodes that are not included in the active routing topology. This is made more
complex when using some form of Traffic Engineering (TE) based routing.
• network and element discovery and cataloging is not clearly defined using IP troubleshooting tools.
There is a need for Layer 2 equivalents to manage and troubleshoot native Layer 2 Ethernet networks. With
these tools, you can identify, isolate, and repair faults quickly and easily, which reduces operational cost of
running the network. OAM also increases availability and reduces mean time to recovery, which allows for
tighter service-level agreements, resulting in increased revenue for the service provider.
In addition to providing end-to-end OAM in native Layer 2 Ethernet Service Provider/Metro networks,
you can also use CFM to manage and troubleshoot any Layer 2 network including enterprise, datacenter,
and cluster networks.
Maintenance Domains
A CFM maintenance domain is a management space on a network that is owned and operated by a single
management entity. The network administrator assigns a unique maintenance level (0 to 7) to each domain
to define the hierarchical relationship between domains. Domains can touch or nest but cannot overlap or
intersect as that would require management by multiple entities.
CFM divides a network into hierarchical maintenance domains, as shown in the illustration below.